If a leak develops in the system of a recirculating respirator with positive pressure operation, e.g., due to the oxygen mask being put on incorrectly, the time during which the respirator can be used is greatly reduced by the great loss of oxygen caused by the over pressure conditions. This may lead to a dangerous situation for the user of the respirator. If the device switches over from positive pressure operation to normal operation in case of a leak, the loss of oxygen will be reduced.
Such a respirator has been known from West German Patent No. DE-PS 32,29,240. This device has a blocking mechanism which eliminates the positive pressure operation and is actuated automatically in case of a leak. This blocking mechanism is designed as a cylinder-piston unit, wherein the movement of the piston is coupled with the movement of the breathing bag. At the same time, the force applied by the piston to the breathing bag via a lever generates the over pressure in the system. The cylinder-piston unit has an outlet valve which opens when a defined stroke position of the piston is exceeded, so that the over pressure in a chamber above the piston is eliminated and the further supply of pressurized oxygen is blocked by an intake valve responding to the pressure reduction in the chamber. As a result, the piston no longer applies a force to the breathing bag, and the device operates in the normal standard-pressure mode. When another defined stroke position of the piston is exceeded, the outlet valve is closed and the intake valve is opened, so that the force for inducing the positive pressure operation is again available. The complicated mechanical design with many sealing members, the lack of an unambiguous recognition of a leak from a single, defined position of the breathing bag, and the lack of information on the existence of a leak for the user of the device are disadvantages of this device.